Rising costs and legal requirements with respect to the disposal of foundry sand in land fills and the like have increased the need for regeneration and reuse of foundry sands. A variety of regenerating processes are known and regeneration apparatus is commercially available and currently in use.
The regeneration of bentonite-containing mixed sands has posed particular problems because of the very large quantity of contaminants which tend to be present in the depleted foundry sand. At the present time, thermo-mechanical regeneration is favored as is described, for example, in EP 0 343 272 A1. In this thermo-mechanical process, the sand is initially annealed in a thermal treatment stage at temperatures from 500.degree. to 900.degree. C. and, after an appropriate cooling, is fed batchwise into a friction or grinding machine in which the dead-burned binder residue which has not been volatized by the annealing process is abraded from the grains of sand by rotating transverse arms. Compressed air blown through the sand filling in the machine from time to time carries away the dust which is liberated by the abrasion process.
Thermal regeneration treatments, however, have not been fully acceptable heretofore because of the cost and time required.
The high capital cost, operating and maintenance costs of earlier apparatus for the regeneration of foundry sand have made many of the earlier systems unsuitable or unacceptable for small and medium-size foundries and have resulted in the need to transport foundry sand from the foundry or to return the foundry sand after treatment to the foundry with expensive transport processes.
Old sand may have heat values in the form of carbon containing components and some bentonite as valuable constituents which are dead burned and lost. Sand grains can fracture as a result of the temperature variations and can be transformed to waste. The amount of waste may thereby be increased beyond that which is desirable or acceptable and the grain spectrum or particle size distribution in the foundry sand may be varied in an impermissible manner. Environmental problems arise from the production of heat and carbon dioxide by the foundry in the regeneration of foundry sand. Restrictions in the amounts of heat and CO.sub.2 which can be liberated by a particular foundry can limit foundry furnace operations in other respects.
In more recent publications, therefore, regeneration processes have been described in which the dead burning of the entire bulk of the old sand can be avoided (see DE 41 06 736 A1, DE 41 06 737 A1, DE 41 21 765 A1 and EP 0 465 778 A2).
When, however, the thermal regeneration step is eliminated, the requirements for a mechanical regenerating machine are significantly higher than with earlier machines since the mechanical operation must satisfy the full need for cleaning the sand grains.
Investigation into commercially available machines has shown that these machines have a variety of drawbacks and disadvantages which limit the quality of regeneration or only allow satisfactory regeneration after long processing times.
Impingement-type cleaning operations in which the sand grains are entrained in high velocity air jets and the stream is directed against impingement baffles, consume relatively too much compressed air and generate large amounts of residues because of the rupture of the sand grains.
Rotary drums with plural drives and strippers are expensive when they must be designed to accommodate hot depleted foundry sand and to tolerate the high degree of wear and the tendency to breakdown which thus results.
With grinding machines which have commonly been used after a thermal treatment stage, the removal of dust is generally effected either by transverse air which can entrain the dust only over the batch in the vessel, or by means of compressed air. When compressed air, however, is fed to the grinding machine of EP 0 343 272 A1 by a multiplicity of nozzles at the upper machine bottom, there is formed in the region of the transverse arms, a fluidized bed. With sand which contains active clay, there is insufficient abrasion which deterimentally affects the suitability of the regenerated product for the production of foundry cores.
To overcome this problem, machine operating times are increased or the amounts of materials passed through the machine are reduced.
Furthermore, the abrasion does not always affect the entire charge in the machine and it is found that at the bottom and side wall corners of the machine there are dead zones where untreated sand can accumulate, the untreated sand serving to contaminate the regenerated product.